Method and apparatus for forming wire from molten material



37, .5. L. ENG-EILKE ET AL 3347,95

METHOD AND APPARATUS FOR FORMING WIRE FROM MOLTEN MATERiAL Filed Oct. 8, 1964 MOLTEN WIRE-FORMING MATERIAL LIQUlD MOLD STREAM SOLID WIRE John L. Engelke. Philip C. Johnson Barry A. S'rein Attorney United States Patent 3,347,959 METHOD AND APPARATUS FOR FORMING WIRE FROM MOLTEN MATERIAL John L. Engelke, Arlington, Philip C. Johnson, Belmont, and Barry A. tein, Brookline, Mass., assignors to Arthur 1). Little, Cambridge, Mass., a corporation of Massachusetts Filed Oct. 8, 1964, Ser. No. 402,394 Claims. (Cl. 264-37) This invention relates to method and apparatus for forming wires and filaments, and more particularly to forming wires and filaments from a material which has a relatively sharp melting point and can be handled in a molten condition.

Although the method and apparatus of this invention are particularly suited to the formation of metal Wires and filaments having diameters ranging from a fraction of a thousandth of an inch to about one-eighth inch, it is not limited to this particular use. However, for convenience of presentation it will be described in terms of forming metal wires and of the advantages which ,are to be gained in metal wire formation.

Extremely large quantities of aluminum, copper and ferrous alloys are used in the form of wires. Aluminum wires are used for electrical conductors, screening, filters, and some screw machine products. Copper wires are used in large tonnages for electrical conductors and in smaller quantities for screening and similar products. Ferrous alloy wires in various sizes are used for making a vast assortment of items from paper clips to spring wire.

All metal wire is now manufactured by the mechanical reduction of either rods or ingots. The mechanical working which gives rise to the various Wires thus made involves several steps ordinarily including hot rolling, cold drawing or rolling, and annealing. The necessity for such extensive mechanical working with its associated equipment is responsible for a substantial increase in price between the ingot and the finished wire product.

Several other alternatives for forming metal wires have been proposed, but are not now in use. One of these is the casting of the molten material directly into a solid mold with solidification taking place as the material advances through the length of the mold out to the production line.

lthough this technique can be applied to produce billets in the range of an inch or so thick, it has not been feasible to extend it to significantly smaller sizes because of the increasing friction and drag on the Walls and the increased energy requirements for forcing the metals through the mold. A second method is that of ejecting a stream of molten material from a nozzle into a gaseous atmosphere. This requires maintaining the thread of liquid for a sufiicient time for the filament thus formed to solidify and thus it avoids the difiiculties associated with the use of the solid mold. However, the surface tension of the molten metal tends to break the filament into spherical particles. This requires then that the molten material be ejected with high velocity which in turn introduces problems in heat transfer. For any reasonably sized wire, the amount of heat to be extracted is too great to make this method practical.

It is therefore an object of this invention to provide method and apparatus for rapidly forming wires or filaments from molten materials, the materials being those which exhibit a relatively sharp melting point. It is another object of this invention to provide method and apparatus of the character described which is particularly well-adapted for making metal wires up to about oneeighth inch in diameter. It is another object to provide such a method which is capable of materially reducing the cost of converting metals to wire form. It is yet another object to provide a method of forming metal Wires 3,3413% Patented Oct. 17, 1967 which are capable of improved quality control. Other in part be apparent hereinafter.

The invention accordingly comprises the several steps and the relation of one or more such steps with respect to each of the others, and the apparatus embodying features of construction, combination of elements and arrangement of parts which are adapted to efiiect such steps, all as exemplified in the following detailed disclosure. The scope of the invention will be indicated in the claims.

For a fuller understanding of the method and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing in which FIG. 1 is a cross-sectional view of the apparatus suitable for carrying out the method of this invention; and

FIG. 2 is a schematic diagram showing the entire apparatus including fluid supply, etc.

By the method of this invention wires are cast from a melt by using a continuously flowing laminar stream of liquid as the mold. The molten material to be cast is continuously introduced in the form of a liquid molten filament into the liquid mold and is substantially coaxial therewith. The liquid mold surrounds the molten filament and its velocity is maintained greater than that at which the molten filament is being advanced. Moreover the velocity of the liquid mold stream is increased at least to the point where suificient heat transfer has been effected to solidify the molten filament to form the wire. By maintaining the velocity of the mold stream greater than the wire-forming molten filament, the diameter of the wire forming filament is reduced by the action of viscous shear forces at the liquid-liquid interface. By controlling the liquid mold stream velocity and the rate at which it is increased, it is possible to control the diameter of the wire formed and substantially to eliminate the control over the wire diameter which might otherwise be exercised by the diameter of the nozzle through which the molten filament is originally ejected. Thus variation in nozzle diameter brought about through erosion or other factors does not affect the final diameter of the wire and need not be continuously monitored or corrected.

Before describing in detail the manner in which the method of this invention forms fine wires of -a desired diameter it will be helpful to describe in some detail a typical apparatus in which the method of this invention may be performed.

In FIG. 1 there is a cross-sectional drawing of the wire-forming apparatus. It will be seen to consist of an outer tubing 10 which has a tapered section 11 and an extended lower narrow section 12. Within this tubing flows a liquid stream 14 which is the mold form used. As the stream enters the tapered section 11, it is accelerated by virtue of the narrowing of the cross-section through which it flows. Positioned within the outer tubing is a feed tube 16 through which the molten, wire-forming material 17 flows and is ejected from the nozzle 18 to form a liquid stream 19 which upon cooling becomes the solid wire 20. Coils 21, adapted to circulate a coolant, are in thermal contact with the external wall of tubing 10 in the area of solidification.

FIG. 2 is a schematic drawing of the apparatus of this invention as it is incorporated in one embodiment With auxiliary equipment. The Wire-forming element of FIG. 1 is generally indicated by the numeral 24. It has associated with it means for supplying molten metal 25 which is in fluid communication with the wire-forming element 24 through a suitable conduit 26 which has valve means 27, a pump 28, and if desired an auxiliary heater 2.9. The wire 20 as formed is passed around a guide roll 32 to a take-up roll 33.

The liquid which is to form the liquid mold stream is supplied from reservoir 35 by way of conduit 36, which contains a valve 37 and a pump 38, into the wire-forming element 24. This liquid stream is then recovered from the bottom of the wire-forming element 24 through a conduit 39 which has associated with it a pump 40 and if desired clean up or auxiliary heating system 41.

It is now possible to look more closely at the mechanism by which wires are formed in accordance with the method of this invention. The molten metal which is to form the solid wire is introduced into the feed tube 16 (FIG. 1) at a pressure greater than that of the local pressure in the liquid mold stream. The ratio of nozzle opening diameter to finished wire diameter is not critical over a wide range since the actual diameter of the finished wire Will be determined very largely by the relative velocities of the two liquid streams. This means of course that the diameter and even the shape of the nozzle opening can be varied and can change throughout operation Without appreciably changing the finished Wire diameter. This in turn means that with careful control of the two stream velocities it is possible to obtain good quality control over the wire diameter.

The molten liquid filament which is to form the wire is positioned within the liquid molten stream so that there exists no appreciable velocity gradient in the molten stream across the filament at any one cross-section. Although it will usually be convenient to inject but one wireforming molten filament into the liquid mold stream, it is possible if the stream is large enough in diameter to use it to mold more than one wire at a time.

The mold form which, as defined above, is a stream of laminarly flowing liquid is essentially inert to and unreactive with the molten metal filament. The velocity of the liquid mold stream must be greater than that of the liquid filament and it must be increasing up to the point in space where solidification of the molten filament takes place to form the wire.

With the surrounding liquid mold stream travelling at a higher velocity than the molten filament, the momentum of the outer liquid mold stream acts to resist lateral defiection. If the two liquids wet each other, then in order for a neck to develop, the molecules of the outer liquid mold stream must be deflected from their course. Their momentum acts to resist such deflections, and therefore acts in opposition to the interfacial tension. If the materials do not wet, then cavitation can occur, but this creates a region of reduced pressure which equally opposes the necking forces. In the limit, this stress is about atmospheric pressure and a simple calculation using the equation, d='y/1rP where d is the diameter of the filament, P is pressure and -y is surface tension, indicates that the minimum stable size of a filament is approximately 0.0003 cm. in diameter for a metal such as aluminum with a surface tension of approximately 500 dynes/ cm. For the case of organic materials, this minimum diameter is nearly an order of magnitude less.

It is of course necessary that the outer liquid mold stream always be in laminar flow. If it is turbulent, the necessary uniform momentum gradient does not exist. Using the usual criterion that the Reynolds number must be less than 2100 in order to assure laminanfiow, it can be shown that maximum velocities greater than 2000 feet per minute can be tolerated for typical surrounding liquids, such as molten salts flowing through an orifice with a diameter less than A inch. The desired ratio of filament diameter to the surrounding liquid mold stream diameter is primarily related, on the one hand, to the extent to which heat transfer from the walls is desired, and on the other hand, to the extent to which small size orifices would present difficult manufacturing problems and sharply increased pumping costs. In any event, since the filament will be accelerated into the orifice by the surrounding liquid mold stream velocities on the order of 2000 ft. per minute can be achieved in a laminar flow regime. This is the same order of magnitude as the speeds utilized in the production of fine wire by wire drawing and hence this method is competitive with wire drawing as far as speed of formation is concerned.

The fact that the combined liquid streams are formed at the entry to a tapered section, is responsible for two of the most important aspects of the method of this invention. In the first place, the taper may be used to perform a liquid analogy to the drawing process, in which the inner filament stream is reduced in diameter by the viscous shear acting at the interface between the two liquids. This liquid drawing process which, of course, is a function of the particular geometry and velocities, is responsible for the fact that very fine filaments can be formed even from a stream having a much larger initial diameter. It is, therefore, not necessary to rely on a precise diameter of shape of orifice in order to control the diameter of the final filament. Moreover, this implies that small changes in nozzle diameter such as might be caused by erosion, will not necessarily affect the diameter of the finished product. Finally the fact that the liquid mold itself is used to reduce the diameter of the wire means that it is possible to adjust the diameter over a reasonable range merely by adjusting the fiow rates, temperature profiles, and the like.

The second major aspect with which the tapered section is concerned is related to the fact that the outer velocity of the surrounding liquid mold stream must be greater than that of the inner liquid filament. By proper design of the taper, the change in velocity of the outer stream as it progresses can be controlled as desired. It is not necessary that the taper be uniform throughout.

Solidification of the filament to form the wire may be achieved by one of two mechanisms or a combination of both. Thus heat may be transferred from the filament to the liquid mold stream which is at a lower temperature than the melting point of the filament material. Heat may also be transferred from the filament through the confining liquid to the walls of the apparatus which may be cooled by the circulation of a coolant through coils 21 (FIG. 1). Heat may also be transferred by a combination of these processes which in any case is based primarily on conduction. It will be appreciated that reliance upon heat transfer through conduction in a liquid gives rise to much greater heat transfer coefficients than could be obtained for example through conduction or convection using a gas in place of the liquid.

It will be preferable to achieve as much of the heat transfer from the filament by conduction to the liquid mold stream. Generally, where smaller-diameter filaments are to be made, the major portion of heat transfer can be accomplished in this manner. This means that the liquid mold stream should have a bulk temperature somewhat below the melting point of the filament material to act as a heat sink of suflicient capacity to cool the molten liquid filament and solidify it. In practice, this amounts to the existence of a gradient between the interface of the two liquids and the bulk of the containing liquid, such that at the interface (near the nozzle) the temperature is at, or slightly above, the melting point, whereas in the bulk, a relatively short distance away, the temperature is a reasonable distance below the melting point, perhaps as much as 50 C. Calculations can be made utilizing the heat capacity of the liquid mold stream and the heat of fusion of the material being cast to determine the relative amount of liquid mold material which would be required to solidify a unit length of filament. For example, if the metal to be cast is aluminum and the liquid mold is a molten salt having a bulk temperature 50 C. below the melting point of the metal, the volume of the liquid mold stream must be approximately 25 times the volume of material being solidified. If the relative velocity is small compared to the overall velocity through the tube, then the diameter of the liquid mold stream must be of the order of 5 times the diameter of the filament.

When the liquid mold stream is used primarily as a heat transfer path, it is necessary to supply an external coolant to maintain the walls of the tube at a temperature much lower than the melting point of the filament. If a parameter L is defined as the distance along the stream which an element of the filament travels while losing its heat of fusion, and the equation is derived which relates this quantity to the physical parameters of the system, there is obtained as a first approximation the following relationship:

where L and d are in cm. and the proportionality constant is obtained by substituting the following reasonable values: velocity, v= cm./sec.; heat of fusion, Q:24O cal./cm. ratio of liquid mold stream to filament diameter, D/d=5; thermal conductivity of fused salts, )\=10- caL/cm. sec. C. and AT=SOO C. To keep L a reasonable length for diameters about 0.01 cm. an increase in the thermal conductivity or the temperature gradient will be required. An increase in the temperature gradient simply implies reducing the thickness of the outer film and this is possible so long as the filament size is above some reasonable minimum size. Thus this manner of heat tranfer for solidification is necessary only for the largerdiameter wires.

Materials which are suitable for forming wires or filaments may be any material which exhibits a relatively sharp melting point and which can be maintained in the molten state without adversely effecting its properties. In addition to metals, other substances such as plastics, organic materials, and some ceramics may be cast into filament form by the method and apparatus of this invention.

The liquid mold material must of course be one which is essentially nonreactive with the material being cast. It must also be thermally stable at a temperature above the melting point of the filament material as well as temperatures below this melting point. For convenience of operation it is preferable that the liquid mold stream material is one which does not react violently with air, water vapor or the available materials of construction. It should of course have a suitable viscosity and thermal conductivity in its liquid form. For example, if wire is to be formed from a metal having a melting point up to and including copper (1985 F.) various molten salts, for example potassium chloride or sodium chloride, may be used as the liquid mold stream. For wire-forming materials which have a melting point lower than about 500 F. it will be possible to use any one of a number of well-known high temperature organic or silicone fluids. For metals, the melting points of which are appreciably greater than that of copper, the liquid molten stream may be a molten metal or a molten salt.

The structure of the filament or wire produced by the method and apparatus of this invention will be essentially that of a cast material. However, because of the fine size and the one dimensional geometry involved and because of the fact that the wire is cast in a protective liquid, the properties of the wire may be unique. It is of course possible to further treat wire or filament made by this method to give it desired metallurgical properties. Thus the wire may be rolled or drawn to produce specific mechanical characteristics.

From the above description of the method and apparatus of this invention it will be seen that there is offered the possibility of forming wires rapidly and with good quality control directly from a molten liquid. The method thus eliminates the need for mechanical working of ingots or rods and thus it is possible to produce wire at a lower cost inasmuch as the intermediate steps are minimized or eliminated. By the simple control of the velocity of two liquid streams it is possible to accurately control the quality of the wire produced both with regard to its mechanical properties and its diameter. Finally the method of this invention lends itself to any material which has a relatively sharp melting point and which includes such diverse materials as metals, plastics, organics, and inorganics.

It will thus be seen that the objectives set forth above, among those made apparent from the preceding description, are essentially attained and since certain changes may be made in carrying out the above methods and in the construction set forth without departing from the scope of the invention, it is intended that all material contained in the above description or shown in the ac companying drawings should be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all the generic and specific features of the inventions herein described and all statements of the scope of the invention which as a matter of language might be said to follow therebetween.

We claim: 1

1. A method of casting a continuous solid filament from a melt, characterized by shaping a continuously moving molten filament to a desired diameter in a mold, said mold comprising a continuously moving liquid stream in laminar fiow completely surrounding said filament and in surface-to-surface contact therewith, the velocity of said mold stream being greater than that of said molten filament and increasing to the point in space where the temperature of said filament is sufficiently reduced to solidify it to form said continuous solid filament.

2. A method of casting a continuous solid filament, comprising the steps of (a) forming a continuously moving molten filament having a predetermined forward velocity;

(b) completely surrounding said molten filament with a liquid mold stream in laminar flow in surface-tosurface contact therewith and having a forward velocity greater than that of said moving molten filament; and

(c) increasing said forward velocity of said liquid mold stream thereby, through the viscous shear forces developed at the interface between said molten filament and said mold stream, to reduce the diameter of said filament to a predetermined value prior to its solidification.

3. A method in accordance with claim 2 wherein the bulk volume and temperature of said liquid mold stream are at levels such that said solidification of said filament is elfected at least in a major portion by transfer of heat to said liquid mold stream.

4. A method in accordance with claim 2 wherein said solidification of said filament is effected at least in a major portion by transfer of heat through said liquid mold stream to an external coolant.

5. A method of casting a continuous solid filament, comprising the steps of (a) ejecting a molten stream of a filament-forming material from a nozzle and imparting to the resulting molten liquid filament a forward velocity;

(b) completely surrounding said molten liquid filament with a liquid mold stream in laminar flow in surface-to-surface contact therewith and having a forward velocity greater than that of said moving molten filament;

(c) increasing said forward velocity of said liquid mold stream thereby, through the viscous shear forces developed at the interface between said molten filament and said mold stream, to reduce the diameter of said filament to a predetermined value;

(d) cooling said molten filament thereby to solidify it to form said continuous solid filament; and

(e) separating said filament from said liquid mold stream.

6. A method in accordance with claim 5 wherein said filament forming material is a metal.

7. A method in accordance With claim wherein said filament forming material is a synthetic resin.

8. A method in accordance with claim 5 further characterized by recycling said liquid mold stream.

9. An apparatus for continuously casting a filament from a material which exhibits a relatively sharp melting point, comprising in combination (a) a first tube terminating in a nozzle and adapted to conduct said material in a molten state under pressure for ejection from said nozzle;

(b) a second tube surrounding said first tube, extending beyond said nozzle, and having a decreasing diameter over that length which extends from at least just above said nozzle to a predetermined point below said nozzle;

(c) means for introducing said molten material under pressure into said first tube; and

(d) means for introducing a liquid mold stream in a laminar pattern into said second tube around said first tube and said filament at a pressure sufficient to 2 impart a velocity to said liquid mold stream greater than that of said filament and to increase said velocity through that length of said second tube having a decreasing diameter. 10. Apparatus in accordance with claim 9 further characterized by having means for withdrawing the liquid forming said mold stream and recirculating it.

References Cited UNITED STATES PATENTS Pazsiczky et al. 1885 Ryan 264204 Manning.

Manning.

Marshall 188 Pedlow et al. 188

Ryan 188 McDermott -5. 188

Dooley 188 Wilke et al. 264-209X Heisterkamp et a1. 264- Cope 18-8 FOREIGN PATENTS 3/ 1962 Great Britain.

0 ALEXANDER H. BRODMERKEL,

Primary Exam iner. 

5. A METHOD OF CASTING A CONTINUOUS SOLID FILAMENT, COMPRISING THE STEPS OF (A) EJECTING A MOLTEN STREAM OF A FILAMENT-FORMING MATERIAL FROM A NOZZLE AND IMPARTNG TO THE RESULTING MOLTEN LIQUID FILAMENT A FORWARD VELOCITY; (B) COMPLETELY SURROUNDING SAID MOLTEN LIQUID FILAMENT WITH A LIQUID MOLD STREAM IN LAMINAR FLOW IN SURFACE-TO-SURFACE CONTACT THEREWITH AND HAVING A FORWARD VELOCITY GREATER THAN THAT OF SAID MOVING MOLTEN FILAMENT; (C) INCREASING SAID FORWARD VELOCITY OF SAID LIQUID MOLD STREAM THEREBY, THROUGH THE VISCUOUS SHEAR FORCES DEVELOPED AT THE INTERFACE BETWEEN SAID MOLTEN FILAMENT AND SAID MOLD STREAM, TO REDUCE THE DIAMETER OF SAID FILAMENT TO A PREDETERMINED VALUE; (D) COOLING SAID MOLTEN FILAMENT THEREBY TO SOLIDFY IT TO FORM SAID CONTINUOUS SOLID FILAMENT; AND (E) SEPARATING SAID FILAMENT FROM SAID LIQUID MOLD STREAM.
 8. A METHOD IN ACCORDANCE WITH CLAIM 5 FURTHER CHARACTERIZED BY RECYCLING SAID LIQUID MOLD STREAM. 